Generally speaking, a conventional magnetic motor includes two pieces that move relative to each other. Each of the two pieces includes some means of generating a magnetic field. The interaction between the magnetic fields generated by each of the pieces forces the pieces to move relative to each other. Usually, the magnetic field of at least one of the pieces will be selectively adjusted over time so that, as the relative spatial relationship of the pieces changes over time, the magnetic fields of the respective pieces will continue to interact to continue to activate relative motion in a desired direction.
Usually at least one of the pieces of the magnetic motor will employ one or more electromagnet(s), such as an electromagnetic coil, to generate its magnetic field(s). By using an electromagnetic piece, the timing of current supplied to the electromagnet(s) can be used to control the direction and strength of the magnetic fields with respect to time. By carefully controlling the electromagnetic piece's magnetic field as its counterpart piece moves, the magnetic field will pull and/or push the two counterpart pieces into relative motion. As the counterpart pieces continue in their relative motion, the direction and/or magnitude of the current in the electromagnet(s) can be changed so that the new magnetic field of the electromagnet(s) will continue to force the desired relative motion.
There are various geometries for magnetic motors. One geometry is the rotary magnetic motor. In a rotary magnetic motor, a rotor piece is driven to rotate relative to a stator piece. Another geometry is the linear magnetic motor. In a linear magnetic motor, a shaft member is driven to move linearly (that is, as a straight line translation) with respect to a stator piece.
In one type of linear magnetic motor, an elongated shaft member is at least partially surrounded by the stator and is constrained radially by a bearing to move linearly within the stator. Generally the bearing housing and stator are fixed relative to each other and can therefore be thought of as a subassembly.
FIGS. 1 and 2 show typical prior art linear magnetic motor 100, including shaft 102, stator 104 and bearings 106. Shaft 102 generates magnetic fields by virtue of having a series of built-in permanent magnets 110. Stator 104 generates magnetic fields through a series of annular magnetic coils 105. By timing the flow of current in the coils with respect to the position and/or momentum of shaft 102, the interaction of magnetic forces from the shaft and from the stator will actuate the shaft to move. More particularly, the shaft is constrained, by bearings 106, to move linearly in the direction of arrow D.
FIG. 2 shows a more detailed view of shaft 102 and one of the magnetic fields that it generates. Shaft 102 includes sleeve 109, annular, permanent magnet 110, pole pieces 112 and core 114. In this assembly, maximizing the magnetic force on the shaft will tend to advantageously maximize the thrust of the linear motor. In order to maximize the magnetic force on the shaft, the magnetic field of permanent magnet 110 should cause as much magnetic flux density as possible linking stator 104 and shaft pole pieces 112.
There are several variables that control the magnitude of the flux density in the vicinity of the stator. One variable is the strength of permanent magnet 110. For more thrust, the strength of magnet 110 should be increased as much as possible and/or as much as is cost effective (without causing saturation).
As shown in FIG. 2, another variable that has an influence on the flux density is the size of the effective air gap G. As shown in FIG. 2, the effective air gap G in this example is the sum of the actual air gap 108 and the thickness of non-magnetic sleeve 109. Some actual air gap is needed to prevent the shaft from rubbing against the non load-bearing surfaces of the stator poles. On the other hand, decreasing this air gap, without entirely eliminating it, will advantageously cause the field of magnet 110 to have greater flux density in the vicinity of the stator due to the increased proximity between magnet 110 and the stator. As flux density from magnet 110 in the vicinity of the stator increases, increased interaction of the magnetic fields results in increased force on the shaft, increased attendant actuation of the shaft and increased motor thrust.
Yet another variable affecting magnetic flux density in the vicinity of the stator is the flux density located across the effective air gap. As shown in FIG. 2, there are generally three paths A, B, C for the magnetic field of magnet 110. While magnet paths are generally circuits, it is noted that the magnetic “paths” referred to in this document refer to the portion of the magnetic circuit that lies outside of the magnet itself.
Path A passes through sleeve 109, which is part of the effective air gap. Path B passes through actual air gap 108, which is also part of the effective air gap. Path C passes through the stator. Permanent magnets are generally limited in the maximum amount of magnetic flux that they are capable of outputting. For an annular magnet of finite flux output capability, greater magnetic flux along paths A and B reduces the flux available for path C. As explained above, it is flux density of path C (that is, flux that reaches the vicinity of the stator) that contributes to motor thrust.
Sleeve 109 is conventionally made from materials that have a low magnetic permeability. The non-magnetic nature of sleeve 109 works to minimize flux along sleeve 109 though path A. Nevertheless, some relatively small amount of magnetic flux is generally “lost” along path A. To represent this lost flux, a solitary dashed flux line is shown passing along and through the sleeve in FIG. 2. Because only a small fraction of the total flux is lost along path A, a higher portion of the total flux generated by magnet 110 will be directed through path C into the vicinity of the stator.
Because actual air gap 108 is made of air, this potential flux leakage path B has extremely low permeability (the relative permeability of air equals 1.0) and no substantial remanent magnetization. Since the path B leakage flux is small and is primarily a result of sleeve 109, no dashed flux lines are shown along actual air gap 108 at the upper half of FIG. 2.
Sleeve 109 provides a bearing surface to slidably mate with bearing 106 as bearing 106 radially constrains the linear motion of shaft 102. If no sleeve were present, then the permanent magnets and the intermediate pole pieces of shaft 102 would contact the bearing. Because of the limited choice of materials that can be used to make the permanent magnets, and because of physical discontinuities between magnets and pole pieces, the exposed magnets would not generally provide an acceptable bearing surface. This is due to the friction and wear characteristics that a surface of exposed magnets and pole pieces, and any imperfection at the mating surfaces, would have.
Besides providing a relatively smooth and low-friction bearing surface, sleeve 109 also helps provide structural integrity for shaft 102. This can be especially important because the strong permanent magnets 110 can create magnetic attraction toward the stator wall sufficient to deform the entire shaft, absent proper structural support.